1. Field of Invention
The invention relates, generally, to installation and operation of inertial sensors—for example, attitude sensors (gyroscopes) or movement sensors (accelerometers)—on board a vehicle and, more specifically, to a calibration method of an inertial attitude or movement sensor installed in an arbitrary position on board the vehicle and a sensor system of dynamics of a vehicle.
2. Description of Related Art
There is increasingly common use of vehicles that have inertial sensors, such as attitude or acceleration sensors, installed on board to provide accurate indications on the orientation of the vehicle in space and its movement dynamics. Such devices not only help in the operation of on-board driving assistance systems, but are essential for the functionality of other auxiliary systems—including, but not limited to, systems for tracking and recording the movements of the vehicle used in anti-theft devices or in devices, known by the term “black box,” for detecting traffic violations or the dynamics of road accidents.
For this purpose, vehicles can, right from the time of manufacture, include orientation-sensor devices (for example, a three-axis gyroscope) and movement-sensor devices (for example, a three-axis accelerometer) adapted to precisely detect and measure the attitude and movement of the vehicle in three dimensions. If these devices are not foreseen at the time of manufacture of the vehicle, they can advantageously be installed at a later time, as aftermarket installations. Both in the first and, above all, second case, however, it is not easy in all vehicle models to set positioning constraints of the sensor device so that its local measuring axes are aligned with the main axes of the vehicle. Constraining the installation of such a sensor device to its correct orientation with respect to the main axes of the vehicle would require an excessive installation time (for accurate installation and for checking that the constraints and the correspondence of the measurements are respected). In any case, it would not preclude possible errors in configuration and measurement such as to jeopardize the correct operation of the system that uses the measurement data of such a sensor.
For this reason, it is known to foresee the arbitrary arrangement of a sensor device on board a vehicle in the most convenient location in the engine cavity or dashboard (for example, in the most easily accessed free seat) and then carry out its calibration with respect to the axes of the vehicle calculating a conversion matrix (or coordinate-transformation matrix or even rotation matrix) adapted to relate the entities measured in the local reference system with those desired in the main reference system of the vehicle.
WO 02/18873 describes a calibration technique for an accelerometer sensor installed in an arbitrary position on board a vehicle based on directly determining six coefficients of the matrix and calculating the remaining three. The direct determining of the six coefficients of the matrix takes place by comparison between respective “acceleration” values detected by the sensor in the local reference system and corresponding real acceleration measurements referring to the vehicle-coordinate system and obtained through a satellite-positioning system integrated in the vehicle. The calculation of the remaining three coefficients of the matrix is obtained by mathematically imposing orthogonality constraints between the directions of the axes of the reference systems.
Disadvantageously, this mixed approach, which reduces the physical measurements to the minimum and fills in the data of the coordinate-transformation matrix based on pure mathematical orthogonality relationships existing between the axes of the reference systems, results in the propagation of measurement errors suffered by the six measurements also on the three items of data collected and causes the risk of concentrating an excessive error on one of the measurement axes, which would significantly compromise the functionality of the application that is based on the accelerometer data of the vehicle.
Thus, there is a need in the related art for a method for calibrating an inertial sensor installed in an arbitrary position on board a vehicle. More specifically, there is a need in the related art for such a calibration method that does not result in propagation of measurement errors, not cause risk of concentrating an excessive error on any measurement axis, and not significantly compromise functionality of application based on accelerometer data of the vehicle. There is a need in the related art also for a sensor system of dynamics of the vehicle able to be installed on board in an arbitrary position.